2,4-endo,endo-Dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one (CAS NO.: ), which is known as 8-Oxabicyclo[3.2.1]oct-6-en-3-one, 2,4-dimethyl-, (endo,endo)-, could be produced through the following synthetic routes.

A. 2-Chloropentan-3-one (CAS NO.: ), 2. A 500-mL, two-necked, round-bottomed flask, containing a magnetic stirring bar, is equipped with a 100-mL pressure-equalizing addition funnel and a reflux condenser fitted with a calcium chloride trap. The flask is charged with 85 mL (69.40 g, 0.80 mol) of pentan-3-one 1 and 200 mL of carbon tetrachloride and the mixture is heated to 45°C in an oil bath. The addition funnel is charged with 71 mL (0.88 mol, 1.1 mol equiv) of sulfuryl chloride that is added dropwise over a period of 2 hr. The resulting mixture is stirred for 3 hr at 45°C, carbon tetrachloride is removed by distillation under atmospheric pressure at 85°C and the residue is purified by distillation under reduced pressure. After a forerun at 65-80°C (62 mm) containing mainly 3-pentanone , 2-chloropentan-3-one 2 is collected (bp 80-102°C, 62 mm) as a pale yellow liquid (77.0 g, 80%).

B. 2,4-endo,endo-Dimethyl-8-oxabicyclo[3.2.1]oct-6-en-3-one (CAS NO.: ), 3. A 500-mL round-bottomed flask is equipped with a magnetic stirring bar and a 50-mL pressure-equalizing addition funnel. The flask is charged with 15.0 g (0.12 mol) of 2, 36.1 mL (0.50 mol, 4 mol equiv) of furan and 125 mL of distilled water. The mixture is stirred vigorously at room temperature and triethylamine (18.05 mL, 0.13 mol, 1.05 mol equiv) is placed in the addition funnel and added dropwise to the reaction over a period of 30 min. The reaction mixture is stirred for 12 hr and quenched by adding a saturated solution of ammonium chloride (50 mL). The mixture is poured into a 500-mL separatory funnel. The layers are separated and the aqueous layer is extracted with dichloromethane (3 × 50 mL). The combined organic layers are washed with brine, dried over anhydrous magnesium sulfate , and the organic layer is filtered and concentrated under reduced pressure. The crude reaction mixture is resubjected to the reaction conditions by adding additional furan and triethylamine (1.05 mol equiv based on the amount of unreacted starting material as determined by 1H NMR). After 5 hr of vigorous stirring at room temperature, the reaction is quenched by adding a saturated solution of ammonium chloride (50 mL). The layers are separated, the aqueous layer is extracted with dichloromethane (3 × 25 mL) and the combined organic layers are washed with brine and dried over anhydrous magnesium sulfate . After filtration, the solvent is removed with a rotatory evaporator and the crude reaction mixture is dried for 2 hr on a high vacuum pump. The oil is cooled to -20°C in the freezer overnight and pale yellow crystals are filtered on a Buchner funnel and washed with cold pentane . The mother liquors are concentrated, placed on a high vacuum pump for 5 hr, and cooled in the freezer. The procedure is repeated once more so that a total of 6.10 g (33%) of pure 3 is obtained.

The filtrate is concentrated under reduced pressure and the residue is purified by flash column chromatography on silica gel [500 mL, 230-400 mesh, eluted sequentially with hexane/ethyl acetate (90% to 70%)] to afford the cycloadduct 3 as a white powder (2.25 g, 12%); total 8.35 g (45%).